1. Field of the Invention:
The present invention relates to an equalizing circuit for correcting the frequency characteristics and level of signals being transmitted through a transmission circuit.
2. Description of the Prior Art:
FIG. 2 is a circuit diagram showing the constitution of an exemplary conventional equalizing circuit. Shown in FIG. 2 are an integrated circuit LS12 in a package form, and the terminals T1 to T5 of the integrated circuit LS12. A high-pass circuit is constructed by connecting the positive terminal of an operational amplifier OP1 to the terminal T1 for receiving an input signal to be amplified, connecting the negative terminal of the operational amplifier OP1 through a resistance R1 to analog switches S1 to Sn for grounding the negative terminal selectively through capacitors C1 to Cn, respectively, and connecting the output terminal of the operational amplifier OP1 through a variable resistance R2 to the negative terminal. The output of the operational amplifier OP1 is provided as an amplified signal through a low-pass circuit LPF2, an operational amplifier OP2 for eliminating out-band high frequency noise, and an operational amplifier OP3 for compensating for a loss in the signal level attributable to the length of the transmission circuit. A peak detecting circuit PKDET2 detects the peak level of the output signal of the operational amplifier OP3, regulates the gain of the operational amplifier OP1 by controlling the variable resistance R2 on the basis of the result of its detection, actuates the analog switches S1 to Sn selectively and regulates a feedback variable resistance R3 for the operational amplifier OP3 to carry out the precise correction of the frequency and signal level, which will be described hereinafter.
FIGS. 3A and 3B are graphs showing the characteristics of a circuit connected to the terminal T1. FIG. 3A shows the variation of the frequency characteristics with the length of the circuit; the signal level drops in a high frequency range as the length of the circuit increases. FIG. 3B shows the variation of the frequency characteristics with circuit length for circuit losses; the level drops due to circuit loss in the entire frequency range.
To equalize the circuit, the equalizing circuit needs to have correction characteristics as shown in FIG. 4A to correct the frequency characteristics of FIG. 3A, and to have correction characteristics as shown in FIG. 4B to correct the frequency characteristics of FIG. 3B.
FIGS. 5A, 5B and 5C show the control characteristics of the equalizing circuit of FIG. 2. As shown in FIG. 5A, the cut-off frequency is variable through the selective control of the analog switches S1 to Sn. As shown in FIG. 5B, the low-pass circuit LPF2 cuts off the high frequency range. FIG. 5C shows the resultant correction characteristics.
The foregoing conventional equalizing circuit needs many parts in addition to the integrated circuit LS12, and hence the integral circuit LS12 needs to be provided with many terminals. Furthermore, inevitable manufacturing irregularity in the resistances of the analog switches S1 to Sn causes the characteristics of the high-pass circuit including the operational amplifier OP1 to vary.